Math.easeInOutQuad = function(t, b, c, d) {
	// eslint-disable-next-line no-param-reassign
	t /= d / 2;
	if (t < 1) {
		return c / 2 * t * t + b;
	}
	// eslint-disable-next-line no-param-reassign
	t--;
	return -c / 2 * (t * (t - 2) - 1) + b;
};

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (function() {
	return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function(callback) { window.setTimeout(callback, 1000 / 60); };
})();

/**
 * Because it's so fucking difficult to detect the scrolling element, just move them all
 * @param {number} amount
 */
function move(amount) {
	document.documentElement.scrollTop = amount;
	document.body.parentNode.scrollTop = amount;
	document.body.scrollTop = amount;
}

function position() {
	return document.documentElement.scrollTop || document.body.parentNode.scrollTop || document.body.scrollTop;
}

/**
 * @param {number} to
 * @param {number} duration
 * @param {Function} callback
 */
export function scrollTo(to, duration, callback) {
	const start = position();
	const change = to - start;
	const increment = 20;
	let currentTime = 0;
	// eslint-disable-next-line no-param-reassign
	duration = (typeof (duration) === 'undefined') ? 500 : duration;
	const animateScroll = function() {
		// increment the time
		currentTime += increment;
		// find the value with the quadratic in-out easing function
		const val = Math.easeInOutQuad(currentTime, start, change, duration);
		// move the document.body
		move(val);
		// do the animation unless its over
		if (currentTime < duration) {
			requestAnimFrame(animateScroll);
		} else {
			if (callback && typeof (callback) === 'function') {
				// the animation is done so lets callback
				callback();
			}
		}
	};
	animateScroll();
}
